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Mu-12 is the true form of Noel Vermillion. Through Yūki Terumi's ambitions, she fused with the true Azure, and awoke as the Sword of the Godslayer Kusanagi, someone who can possibly kill a God.

Mu was made in the 5th Hierarchical City of Ibukido. As the twelfth Murakumo Unit to be made; she was christened as Mu -No.12-. Made by Sector Seven to become the heart for the Black Beast and to fuse with Hazama, the body, she was to be made in the middle of the Ikaruga Civil War to become the ultimate weapon for Sector Seven to use against the NOL. However, this plan was thwarted by Takamagahara System as they fired Gigant: Take-Mikazuchi upon the city, utterly obliterating every living being within the blast radius. Mu, in the stage of being smelted, and thus, in a cocoon, was saved from the same fate as the others, but left without an identity. She was later found and then subsequently adopted by Edgar and Claire Vermillion and christened as Noel Vermillion.

Thanks to Rachel Alucard, Noel regains Mu's powers and abilities in order to save Tsubaki Yayoi with the power of the Azure. She no longer possesses the same cold, destructive personality she had after her initial awakening, retaining Noel's personality and memories instead.

While Noel Vermillion is the definition of a "shrinking violet", Mu -No.12- is completely the opposite: she becomes emotionless, and wants the world to be destroyed. She only acts based on Terumi's orders, and she believes that the world "hates her", and is filled with nothing but lies. However, after Noel gained control over her powers and transformation into Mu, she now retains her normal personality and emotions when in Mu's form. Upon transforming willingly, Noel noticeably becomes more "in control" of her emotions, as opposed to when she felt as though she blacked out when fighting using Bolverk. She is also shown to be more confident and outspoken, though she still has some degree of shyness and embarrassment regarding her rather revealing attire, particularly when facing Ragna, especially facing Kagura.

Being Noel's true form, Mu's physical appearance remains the same, with the exception of her eyes having changed to a soulless blue featuring tiny pupils, and her long blond hair with a distinct fray. Outside of combat, her outfit is similar to Nu -No.13-'s, but with a more elaborate and advanced design, featuring lights, thigh-high toeless socks, and blue nail polish on her hands and feet. She wears an armored headplate featuring long horns like Hakumen's. When in battle, she dons armor similar to Nu and Lambda -No.11- with the same general design and floating blades. However, unlike the former two, her armor shows off much of her skin, retaining little of Noel's NOL outfit, which seems to have disintegrated. It also features large pauldrons that contain what appear to be her Azure Grimoire. Her pauldrons bear a striking resemblance to the pauldrons of Hakumen's armor, except with extensions for the Azure Grimoire.

As a child, she was wearing a blue experimental robe. Her hair retains the distinct fray. This version is also seen when Mu is affected by Amane's Astral Heat.

In Chronophantasma, as Noel is now in full control of her powers, Mu no longer has blank and lifeless pupils, her blue eyes are no longer dulled, and now have definition.

As with her sister units, Mu is a very deadly opponent on the battlefield. As the Kusanagi, the Godslayer, she was designed for the sole purpose of destroying the Amaterasu Unit. When fusing with the true Azure, she dons an advanced version of the armor worn by Nu and Lambda. Like them, she has the ability to hover and fly through the air, and can use the disembodied swords behind her to attack and defend herself. She is also able to conjure up an energy barrier by forming the blades into a rhombus-like formation. Mu possesses spatial related powers, preemptively striking opponents by opening wormholes and stabbing her weapons through to the other side and attack them at a distance.

Mu's Drive is Steins Gunner, which involves Mu displacing floating units across the battlefield. These units come in three types depending on the number of blocks, which open up several offensive options, mostly in attacking the opponent from all angles and even reflecting off of each other in order to disorient the opponent's focus. These units will fire a beam of energy when initially deployed, but Mu can fire her own beams into these units to maximize their potency.

Mu's name comes from Mu, the twelfth letter in the Greek Alphabet and serves as the beginning bit of her Murakumo Unit serial number. Mu is also the Japanese reading of the Chinese character wu (無), representing something which does not exist or has no meaning. When written alone, it can represent "the state of oblivion upon death".

12 serves as the final part of her serial number and is used to show that she is the twelfth Murakumo Unit.﻿